1. Field of the Invention
This invention relates to an x-ray exposure apparatus for exposing a pattern on a substrate, such as a wafer or the like, using soft x-rays or the like, and a semiconductor-device manufacturing method using such an apparatus.
2. Description of the Prior Art
X-ray exposure apparatuses have been proposed which use synchrotron radiation (SOR) light or other kinds of soft x-rays, and in which the portion of the optical path of illuminating light which is located before a mask is disposed in a vacuum or in a He atmosphere to reduce the attenuation of x-rays, while the mask, a substrate (such as a wafer or the like) units to carry these components, and the like are disposed in air.
An exposure method which utilizes x-rays as illuminating light is effective for printing a circuit pattern having a fine line width which is less than the limit of the resolution of a reduction projection exposure apparatus which utilizes ultraviolet rays.
In order to realize such a fine line width, each of the factors which cause errors in the accuracy of the line width must be controlled within a predetermined accuracy.
These factors, for example, relate to accuracy in the production of the mask, a resist process for the wafer, and the resolution of the exposure apparatus.
An x-ray exposure apparatus, in general, adopts a so-called proximity exposure method in which an exposure operation is performed while a mask is disposed close to a wafer. In this method, the line width of a pattern to be exposed is influenced, for example, by Fresnel diffraction due to pattern edges of the mask, and the half shadow of illuminating light. The degree of such influence varies in accordance with variations in the amount of exposure, which is the product of the exposure time and the intensity of illuminating light or the absorption power of a resist.
Accordingly, in order to increase the resolution of the exposure apparatus and precisely control the line width of a pattern to be printed, it is necessary to precisely control the amount of exposure.
The specifications of the necessary control accuracy of the amount of exposure of an exposure apparatus are calculated in the following manner.
If the target accuracy of the line width of a pattern is assumed to be .+-.5% for a line width of 0.3 .mu.m, the line width must be controlled within a range of 0.3.times.0.05=0.015 (.mu.m). If the portion of the overall error allocated to the printing accuracy of the exposure apparatus is assumed to be half the overall value, then the permissible variation in printing accuracy equals 0.015.times.1/2=0.0075 (.mu.m). That is, variations in the line width caused by the exposure apparatus must be controlled within 0.0075 .mu.m.
The influence of control accuracy of the amount of exposure on the line width due to Fresnel diffraction and half shadow of the light source of the apparatus is described, for example, in NTT R & D, April 1990, p. 605.
According to the result of experiments in this report, the line width changes 0.002 .mu.m when the amount of exposure changes 1%. This value substantially coincides with the result of calculation made in consideration of Fresnel diffraction and the like.
Accordingly, in order to provide accuracy in the line width of less than 0.0075 .mu.m, the following relationship must be satisfied: EQU 0.2.times..DELTA.D/D&lt;0.0075 EQU .DELTA.D/D &lt;0.0375,
where, .DELTA.D/D is the control accuracy of the amount of exposure.
That is, the control accuracy of the amount of exposure must be controlled within 3.75%.
The following factors which cause errors in the amount of exposure can be considered.
These factors comprise variations in the intensity of the light source, variations in the reflectivity of an x-ray mirror used in an x-ray optical system for expanding the exposure region and selecting x-rays having a predetermined wavelength, variations in the thickness of a partition window between the air and a He atmosphere or a vacuum-tight chamber for guiding illuminating light, variations in the density of He or the air in the optical path of illuminating light due to variations in the temperature or pressure of He or the air, variations in the thickness of a mask membrane, accuracy in the setting of the exposure time, and the like.
Among these factors, variations in the pressure of the air have the following influence.
If x-rays having a wavelength of 10 .ANG. pass a distance of 10 mm in air having a pressure of 1 atm, the intensity of the x-rays is attenuated about 90%. If the pressure changes 1% in this state, the intensity of the x-rays after passing that distance changes about 2.3%.
This value is not a negligible amount since the combined factors which cause errors in the amount of exposure must be controlled within the accuracy of about 3.75%.